Initialize the mutexes used for the NVM and the swflag as MTX_DUPOK in

order to avoid otherwise harmless witness warnings when these are acquired
at the same time and due to both using MTX_NETWORK_LOCK as their type.
The right fix actually would be to use different, descriptive types for
these. However, the latter would require undesirable changes to the shared
code base. Another approach would be to just supply NULL as the type, which
was deemed as less desirable though as it would cause the unique but cryptic
name also to be used for the type and to diverge from the type used by other
network device drivers.

MFC after:	1 week

sys/dev/e1000/e1000_osdep.h

@@ -84,7 +84,8 @@
 /* Mutex used in the shared code */
 #define E1000_MUTEX                     struct mtx
 #define E1000_MUTEX_INIT(mutex)         mtx_init((mutex), #mutex, \
-                                            MTX_NETWORK_LOCK, MTX_DEF)
+                                            MTX_NETWORK_LOCK, \
+                                            MTX_DEF | MTX_DUPOK)
 #define E1000_MUTEX_DESTROY(mutex)      mtx_destroy(mutex)
 #define E1000_MUTEX_LOCK(mutex)         mtx_lock(mutex)
 #define E1000_MUTEX_TRYLOCK(mutex)      mtx_trylock(mutex)